This invention relates to a subscriber multiplexer comprising means for receiving signals transmitted from subscriber interfaces and feeding those signals further to a telephone exchange via a PCM connection, means for feeding signals transmitted from the telephone exchange via a PCM connection further to subscriber interfaces, and measuring devices for checking the condition of a subscriber interface of the subscriber multiplexer. The invention further relates to a telephone system comprising a telephone exchange, and a subscriber multiplexer connected to the telephone exchange by a PCM connection, the multiplexer comprising subscriber interfaces and measuring devices for checking the condition of subscriber interfaces by means of check measurements. The invention further relates to a method for communicating to the telephone exchange the condition of the subscriber interface of a subscriber multiplexer connected to the telephone exchange via a PCM connection.
A subscriber multiplexer is previously known, the subscriber interfaces of which are connected to subscriber stations, such as telephones and similar devices, by means of a converter means and two-wire lines. The converter means adapts, in that case, the subscriber station electrically to the subscriber multiplexer, and carries out the necessary analog-to-digital conversions. The subscriber station multiplexer is connected to the telephone exchange via a PCM (Pulse Code Modulation) connection, in which one channel of the PCM connection is allocated for each subscriber interface. The frame structure of the PCM connection between the subscriber multiplexer and the telephone exchange is typically in accordance with CCITT (The International Telegraph and Telephone Consultative Committee) recommendation G.704, the interface of the subscriber multiplexer and the telephone exchange typically being an interface in accordance with CCITT recommendation Q.512.
The weakness of the above mentioned subscriber multiplexer is the difficulty in monitoring the condition of subscriber interfaces, since monitoring the condition of a wire connected to a subscriber interface requires the visit of a maintenance person in site. The maintenance person connects a separate service terminal including measuring devices to the subscriber multiplexer, subsequent to which the maintenance person detects the condition of the subscriber interface, e.g. by means of measurements of current, voltage and/or insulating resistance. Checking the condition of the interface of a subscriber multiplexer is thus a highly inconvenient and slow measure, and also relatively costly, as it requires a visit of a maintenance person to the site of the interface.